1. FIELD OF THE INVENTION
The present invention relates to wheel support devices for motorcycles.
2. BACKGROUND ART
A rear support structure for a motorcycle is disclosed in Japanese Patent Laid-open No. Hei. 4-325389. As shown in FIGS. 1 and 4 of the above mentioned publication, this technology has an axle shaft 7 with an outer rim 3 formed at one end of a hollow shaft 2, and screw threads 9 formed on the other end. The axle shaft 7 is fixed between a rear fork 10 and fastened thereto by fixedly attaching and tightening a nut 9 onto the threads 9.
When, for example, a sprocket (not shown) is attached to an outer rim 3 side of the axle shaft 7, and this sprocket is driven by a chain, the outer rim 3 side is exposed to a large shearing force, as compared to the threaded side of the axle shaft 7. Moreover, when the sprocket is driven by a chain, the axle shaft 7 is subjected to bending in the driving direction thereof However, since the axle shaft 7 is fastened between a rear fork and set at a fixed width with the nut 8, if the axle shaft 7 is subjected to bending in the driving direction, the head section end is subjected to tensile force at the inner side.
As a result of the shear force in the vicinity of the outer rim 3, and a force in a direction so as to open the head section end towards the outside, both acting on a section of the axle shaft 7 continuous to the outer rim 3, there is a danger of cracks occurring in a base of the outer rim 3.
In order to prevent this cracking, the overall thickness of the hollow shaft 2 is increased, which in turn hinders attempts to manufacture a vehicle light in weight.